Sprint said on Thursday that the iPhone would become available this month on Virgin Mobile USA, its service for prepaid plans, where customers pay for the service as they use it. The cost will be considerably less than the fees contract customers pay monthly to use an iPhone — once you get past the upfront price of the phone itself.

Customers who opt to buy an iPhone with a prepaid plan will have to spend $650 for the iPhone 4S, or $550 for the older iPhone 4. But the baseline $30 monthly fee includes 300 minutes, unlimited data and unlimited text messages. By contrast, AT&T and Verizon no longer offer unlimited data plans, and their contract customers pay upward of $90 a month to use an iPhone.

From personal experience, I have a deep-seated mistrust of prepaid phone plans. Although you gain immediate ownership of your desired device, the costs involved with its sustained use have a distinct tendency toward unwarranted growth.

In a world governed by the intelligence and interconnection of mobile devices, relying upon a non-contractual deal seems destined for trouble. When rolling out new network technologies, optimizations, and general consumer-facing benefits, the prepaid plan is likely the last to gain from any such improvements, and the first to undergo significant overhaul when the market demands it.

In other words, although unlimited data is available now, do not misconstrue this as an element of permanence to any such prepaid plans.